Transcript variant a: 5376 bp. The isoform a lacks the primary proteolytic cleavage site. As a result, the product encoded by this isoform is a membrane bound protein. Transcript variant b: 5460 bp. The transcript isoform b contains the primary proteolytic-cleavage site and encodes a soluble product. Although SCF exists as a monomer, Zhang et al. and Hsu et al. evidenced that dimerisation of SCF has been associated with KIT receptor activation and signal transduction. They demonstrated, through the crystal analysis, that the SCF dimer complex comprises of two SCF monomers with head-to-head interaction in order to form an elongated homo-dimer stabilised by both polar and non polar interactions. Alternative splicing of the SCF transcript results in the inclusion or exclusion of an exon 6 which contains a proteolytic cleavage site, recognised by metalloprotease-9 enzyme that cleaves after an alanine residue (Ala 189) in the extracellular region, producing the 165-aminoacid soluble SCF. There are other proteases that have been suggested to be responsible for cleavage of membrane-bound SCF, as chymase-1, ADAM17 and ADAM33. The splice form that lacks the cleavage site and remains linked to the cell surface, is a result of the alternative splicing within exon 6 which skip the cleavage site for the metalloprotease-9. In total, there are six alternative transcripts of SCF in humans, out of which four encode protein (http://www.ensembl.org).

Protein

Adapted from Johan Lennartsson and Lars Rönnstrand, 2012.

Description

The membrane bound form is a surface molecule of 248 aa, that includes 23 aa of the highly hydrophobic transmembrane domain; the second form corresponds to a soluble protein constituted by the first 165 aa of the extracellular domain released by a posttranslational processing, consisting in a proteolytic cleavage of the mature SCF in the extracellular juxtamembrane region. The full length transcripts encode for a transmembrane precursor of the soluble protein; an alternative splicing that involves the region corresponding to exon 6 of the SCF cDNA, which contains the proteolytic cleavage site, encodes for a surface molecule. Jiang et al. evidenced the crystallized structure of interaction between SCF and c-KIT and revealed the common structure of a bundle of 4 α-helices linked by two intra-molecular disulfide bridges.

Expression

According to description of Bedell et al., the SCF encoding mRNA is characterized by a short 5' untranslated region, a 0.8 kb open reading frame, and by a long 3' untranslated region. In the 5' region, there are three ATG motifs where the last is used as the initiation site. A TATA box consensus sequence (TATAAA) and three overlapping GGCGGG motifs are located at twenty-eight bases upstream of the transcription initiation sites. These are binding sites for the transcription factors TFIID and SP1, respectively. Kobi et al. reported that the POU-homeodomain transcription factor POU3F2, expressed in neurons and in melanoma cells, regulates the SCF promoter through a cluster of four closely spaced binding sites located in the proximal promoter. It should be noted that UVB light is also known to induce expression of SCF in human epidermal cells both on the mRNA level and is soluble as well as membrane-bound SCF, but the mechanism of induction of SCF gene expression by UVB is still unknown. It has been also reported that HIF-1 upregulates the expression of SCF in response to hypoxia as well as to growth factor receptor activation. In Sertoli cells, SCF expression is up-regulated by treatment with follicle stimulating hormone (FSH) through an increasing of cAMP level. SCF transcripts have been found in the cells surrounding kit-positive cells, such as granulosa and Sertoli cells, bone marrow stromal cells and in fibroblasts, keratinocytes and mature granulocytes; SCF expression of peripheral lymphocytes and monocytes is still controversial.

Localisation

Plasma membrane or interstitial space. It is interesting to note that Faber et al. showed that disintegrin and metalloproteinase ADAM10 has an important role in mast cell migration and distribution. In fact, they evidenced that ADAM 10, expressed at high levels by mast cells, is required for SCF-mediated mast cell migration.

Function

SCF/MGF binding of receptor KIT, with tyrosine kinase activity, induces receptor dimerization, autophosphorylation and signal transduction via molecules containing SH2-domains; the soluble and the transmembrane protein have a different biological activity; the soluble form mainly stimulates cellular proliferation; the membrane-bound isoform induces an activation of the receptor more prolonged than the soluble one.

Human mutations are yet unknown in human MGF/SCF gene; mouse mutations at the murine steel (Sl) locus that encodes MGF are known and give rise to deficiencies in pigment cells, germ cells, and blood cells; in particular the steel-Dickie (Sld) mouse has a 4.0-kb intragenic deletion that truncates the Sl coding sequence; Sld mice are only capable of encoding a soluble truncated growth factor that lacks both transmembrane and cytoplasmic domains.

In skin from patients with mastocytosis, MGF was found prevalently free in the dermis and in extracellular spaces between keratinocytes suggesting the presence of a soluble form of the protein; altered distribution of mast cell growth factor in the skin of patients with cutaneous mastocytosis is consistent with abnormal production of the soluble form of the factor, resulting by an increased cleavage of SCF with excessive release of a soluble form from the normally membrane bound form; no sequence abnormalities were detected in MGF mRNA. Janson et al. evidenced that RIN3, a RAS effector, is highly enriched in mast cells, and that is involved in a complex with BIN2, a membrane binding protein implicated in endocytosis. They also demonstrated that RIN3 negatively regulates KIT internalization process and also that KIT down-regulation is enhanced by RIN3 activity.

Entity

Gynecological tumors

Note

Findings obtained on three cervical carcinomas (ovarian serous adenocarcinoma, small cell carcinoma and ovarian immature teratoma) and two gynecological cancer cell lines (ME180 and HGCM) demonstrate coexpression of c-Kit receptor and SCF; these observations are consistent with the possibility that an autocrine activation of SCF/KIT system might be involved in gynecological malignancies.

SCF is expressed in small cell lung cancer (SCLC); abundant expression of SCF and c-Kit mRNA was seen in 32% of SCLC cell lines and 66% of SCLC tumors; an autocrine mechanism in the pathogenesis of SCLC is strongly suggested.